Circuit for converting dc voltage into three-phase voltage

ABSTRACT

A circuit for converting a DC voltage or a three-phase voltage of variable or constant frequency into a three-phase voltage of a constant other frequency utilizes an intermediate circuit frequency converter. The converter comprises two three-phase thyristor inverters, connected in parallel at the input side. The three-phase outputs of the one three-phase thyristor inverters are connected together via voltage divider chokes. The taps of the voltage divider chokes provide the three outputs of the converter.

Unlted States Patent 1 1 1111 3,781,635 Sauer Dec. 25, 1973 [54] CIRCUITFOR CONVERTING DC VOLTAGE 3,657,633 4/1972 Urish 321/010. 1 INTO THREEPHASE VOLTAGE 3,638,094 1/1972 VeNard 32l/D1G. 1 3,573,602 4/1971 Jensen321/27 R Inventor: Helmut Saver, Nurnberg, Germany 3,628,123 12 1971Rosa et al. 321/9 R [73] Assignee: Siemens Aktiengesellschatt, Berlin,

Germany Primary Examiner--Gerald Goldberg [221 Filed: Jam 18, 1972Attorney-Arthur E. Wilfond et al.

[21] Appl. No.: 218,792

[57] ABSTRACT 30 F'Alt'lP"tDta orelgn pp y a A circuit for converting DCvoltage or a three-phase Jan. 25, 1971 Germany P 21 03 230.4 voltage ofVariabe or constant frequency into a three 'phase voltage of a constantother frequency utilizes an [52] 321/9 321/ gi Egg intermediate circuitfrequency converter. The converter comprises two three-phase thyristorinverters, 2 g g g connected in parallel at the input side. The three- 10 care 321/27 k phase outputs of the one three-phase thyristor invertersare connected together via voltage divider chokes. The taps of thevoltage divider chokes provide the [56] UNITE S SEIFFES SZPFENTS threeoutputs of the converter.

3,568,021 3/1971 Turnbull 321/4 X 1 Claim, 8 Drawing Figures 393 SOURCEW1 FIRST INVERTER SECOND INVERTER CIRCUIT FOR CONVERTING DC VOLTAGE INTOTHREE-PHASE VOLTAGE The present invention relates to a converter. Moreparticularly, the invention relates to a circuit for converting a DCvoltage or a three-phase voltage of variable or constant frequency intoa three-phase voltage of a constant other frequency.

It is customary in the production of a twelve pulse voltage to add thevoltages of two six pulse converters by means of two threephasetransformers. The volume and weight of such converters is considerable.

If the objective is to produce from a three-phase voltage with variablefrequency, a three-phase voltage with constant frequency and, ifpossible, a sinusoidal voltage curve, a space and weight-savingstructure is frequently required for the converters.

The invention emanates from a known intermediate circuit convertercomprising a rectifier and two inverters which are connected in parallelon the input side. Calculations during tests conducted with suchintermediate circuit converters have shown that the addition of thevoltage of the two parallel-connected inverters on the input side byutilizing two three-phase transformers whose space requirement andweight are relatively large, can be omitted and that the three-phasetransformers may be replaced by chokes which weigh less, providedcertain requirements are satisfied.

The principal object of the invention is to provide a converter of theaforedescribed type which overcomes the disadvantages of similar knowntypes of converters.

An object of the invention is to provide a converter of theaforedescribed type which is of light weight and occupies little space.

Another object of the invention is to provide a converter of theaforedescribed type which is efficient, effective and reliable inoperation.

In accordance-with the invention, a circuit for converting a DC voltageor a three-phase voltage of variable or constant frequency into athree-phase voltage of a constant other frequency utilizes anintermediate circuit converter. The converter comprises a rectifier andtwo three-phase thyristor inverters connected in parallel on the inputside. Each of the phase outputs of one inverter is connected via avoltage divider choke to the corresponding phase outputs of the otherinverter. The taps of the chokes provide the outputs of the converter.

The winding ratio of each of the tapped choke windirggsshould be 1 1/1V5 and the inverters should be controlled so that during a rectangularhalf wave of a phase of one inverter, the corresponding half wave of thephase of the other inverter is switched over to the other voltagedirection at the onset and prior to the termination of the halfperiod'for about one-sixth of the time period.

The control of both inverters and the voltage division at the chokewindings produces a voltage between the taps of the choke windingshaving a first occurring upper harmonic which is the 11 th upperharmonic, in accordance with a twelve piilse circiifi'. i

In order that the invention may be readily carried into effect, it willnow be described with reference to the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of an embodiment of the converter of theinvention; and

FIGS. 2a to 2g are graphical presentations of a plurality of voltagesoccurring in the converter of FIG. 1.

FIG. 1 illustrates a three-phase source DO. A rectifier device G isconnected to the three-phase source DO. The rectifier G comprises aplurality of rectifiers G! and smoothing capacitors Cl and-C2. A DCvoltage is thus available at the outputs of the rectifier device G. TheDC voltage may be used as a supply voltage for the inverters W1 and W2.The illustrated converter circuit may thus function as a DC-AC converteror as a DC three-phase current converter.

The inverter W1 has three-phase output terminals u, v and w and theinverter W2 has three-phase output terminals x, y, and z. The phaseoutput u of the inverter W1 is connected to the phase output x of theinverter W2 via a choke K1. The phase outputs v and w of the inverter W]are connected to the phase outputs y and z of the inverter W2 via chokesK2 and K3,.respectively.

The chokes K1, K2 and K3 have taps a1, a2 and a3 connected to outputs R,S and T of the converters. The desired constant output frequency may bederived from the outputs R, S and T. Each of the chokes K1, K2 and K3has a winding ratio kl:k2 selected with respect to its tap in a mannerwhereby it satisfies the relation The inverters W1 and W2 are variablycontrolled in such a manner that during a rectangular half wave of onephase of one inverter, the corresponding half wave of the phase of theother inverter is switched over to the other voltage direction at theonset and prior to the end of the half period duration, for one-sixth ofthe time period. This is illustrated in FIGS. 2a to 23 wherein thedesired voltages are shown.

In FIGS. 2a to 2g the illustrated rectangular voltages are explained bythe accompanying legend wherein, for example U indicates that thevoltage at the terminal u of the inverter W1 is opposite the referencepotential 0; 0 defining the connection point between the two capacitorsC1 and C2 (FIG. 1) of equal capacitance.

The voltage curves of FIGS. 2a and 2b indicate that during the negativehalf wave of the phase u of the inverter W1, phase x of inverter W2isfirst a positive voltage value for 30, equal to one-sixth the halfperiod duration, thereafter for a negative voltage value equal tofour-sixths the half period duration and, tinally, prior to theexpiration of the negative half wave of phase u, a positive voltagevalue for 30, equal to one-sixth the duration of a half period againoccurs at the terminal x. During a positive half period of therectangular voltage at the terminal u, corresponding voltage curvesoccur at the terminal x, while opposite voltage pulses of one-sixth thehalf period duration occur at the beginning and prior to the end of thehalf period.

FIG. 20 illustrates the voltages occurring, for example, at the choke K1(FIG. 1) and FIG. 2d shows the voltage at the component winding kl ofthe choke Kl. FIGS. 2e and 2f show the voltage curve between the outputsR and S toward the point 0 (FIG. 1). By subtracting the voltage U (FIG.2f) from the voltage U (FIG. 2e), the phase to phase voltage U isobtained.

The voltage U which is the output voltage between the terminals R and S(FIG. 1) has the curve shown in FIG. 2g. Thus, a curve for voltage U isprovided where the first-occurring upper harmonic is the 11th upperharmonic having an effective value of oneeleventh of the fundamental.

While the invention has been described by means of a specific exampleand in a specific embodiment, it

'should not be limited thereto, for obvious modificaeach of theinverters having three phase outputs, each of the phase outputs of thefirst inverter being electrically connected to a'corresponding one ofthe phase outputs of the second inverter via one of the chokes; thefirst and second inverters being controlled in a manner to providerectangular output voltages of equal fundamental frequency and ofdifferent curve shape, the output voltage of the first inverter beingformed of positive and negative half waves and the output voltage of thesecond inverter being such that during a rectangular half wave of onephase of the first inverter the corresponding half wave of thecorresponding phase of the second inverter at the beginning and prior tothe end of the half period duration is switched to the other voltagedirection for one-sixth of said time period; and three outputs of thecircuit, each connected to the tap of a corresponding one of the chokes.

1. A circuit for converting a DC voltage into a three-phase voltage,said circuit comprising a first three-phase inverter and a secondthree-phase inverter connected in parallel at their DC voltage inputs;three chokes each comprising of a winding having a tap forming a ratioin the winding and the ratio of each of the tapped choke windings being1/(1 + square root 3) ; each of the inverters having three phaseoutputs, each of the phase outputs of the first inverter beingelectrically connected to a corresponding one of the phase outputs ofthe second inverter via one of the chokes; the first and secondinverters being controlled in a manner to provide rectangular outputvoltages of equal fundamental frequency and of different curve shape,the output voltage of the first inverter being formed of positive andnegative half waves and the output voltage of the second inverter beingsuch that during a rectangular half wave of one phase of the firstinverter the corresponding half wave of the corresponding phase of thesecond inverter at the beginning and prior to the end of the half periodduration is switched to the other voltage direction for one-sixth ofsaid time period; and three outputs of the circuit, each connected tothe tap of a corresponding one of the chokes.